1. Field of the Invention
The present invention relates generally to a drum mixer and, more particularly, but not by way of limitation, to an asphalt drum mixer which has a combined heating and mixing zone for receiving virgin aggregate and recycle aggregate in the preparation of hot mix asphalt.
2. Description of Related Art
U.S. Pat. No. 4,919,538 discloses a counterflow drum mixer having a combined mixing and heating zone. In this type of drum, virgin aggregate is introduced into a drying zone at one end of the drum and is urged through the drum to a mixing zone at the opposite end of the drum. Between the drying zone and the mixing zone is a combustion zone, which includes a burner flame for drying and heating aggregate.
Virgin aggregate is introduced into the drying zone and is urged toward the mixing zone as the drum mixer is rotated. The burner flame produces a hot gas stream which flows toward the drying zone, or counter to the movement of the virgin aggregate.
Within the combustion zone, a combustion housing shrouds the burner flame. An annulus is defined between the drum wall and the combustion housing. In this particular drum mixer, the combustion housing is a series of concentric cylindrical shells having diameters which become successively smaller from the entry end to the exit end of the combustion zone.
Recycle asphalt pavement is introduced into the annulus at the entry end of the combustion zone and hot, dry virgin aggregate is urged into the annulus from the drying zone. Then the virgin aggregate and the recycle asphalt pavement are heated and mixed in the annulus before being transferred into the mixing zone.
One object of asphalt drum mixers is to provide enough heat transfer to the virgin aggregate to remove moisture from the virgin aggregate in an efficient manner. Another object is to introduce the recycle asphalt pavement into the drum mixer at a point where it is not subjected to so much heat that smoke is produced.